The present invention relates to the transfer step in electrostatographic printing, such as xerography, wherein marking material is electrostatically transferred from a charge receptor onto a print sheet.
The basic process steps of electrostatographic printing, such as xerography or ionography, are well known. Typically an electrostatic latent image is created on a charge receptor, which in a typical analog copier or xe2x80x9claser printerxe2x80x9d is known as a photoreceptor. The suitably charged areas on the photoreceptor surface are developed with fine toner particles, creating an image with the toner which is transferred to a print sheet, which is typically a sheet of paper but which could conceivably be any kind of substrate. This transfer is typically carried out by the creation of a xe2x80x9ctransfer zonexe2x80x9d of AC and DC biases where the print sheet is in contact with, or otherwise proximate to, the photoreceptor. In general, the AC bias dislodges the toner particles which were adhering electrostatically to the photoreceptor, while the DC bias, also known as a xe2x80x9cdetack voltage,xe2x80x9d causes the particles to be attracted in imagewise fashion to the print sheet, thus transferring the image from the photoreceptor the print sheet. Devices to create this transfer zone, such as corotrons, are well known.
It has been found, particularly in the design of compact copiers and printers, that the quality of image transfer can vary between a lead edge of a print sheet (i.e., the first edge of the sheet that approaches the photoreceptor) and the trail edge (i.e., the last portion of the sheet to be close to the photoreceptor). Depending on a specific design, there may be any number of reasons for this. For instance, when relatively heavy papers are used, the trail edge of each sheet may not be in the same tight contact with the photoreceptor as the lead edge had been. Also, in a small machine, the trail edge of the sheet may still be in the transfer zone while most of the sheet is in or past the fuser, and mechanical disturbances from the fuser may travel through the print sheet during the last part of the transfer step.
The present invention relates to a method of controlling the transfer step, to obviate the above-mentioned practical difficulties.
U.S. Pat. No. 4,190,348 discloses a xerographic transfer system in which a non-uniform increase in transfer charge is applied to the lead edge of each copy to improve the effective image transfer.
U.S. Pat. No. 5,083,167 discloses a transfer device which supplies a different electric charge amount per area to an end of the transfer material relative to the rest of the transfer material. FIGS. 7 and 9 show how charge is ramped up immediately before a sheet is transferred, and ramped down immediately thereafter.
U.S. Pat. No. 5,287,163 discloses a transfer system in which the transfer bias is progressively increased, in absolute terms, between a leading and trailing edge of a sheet having an image transferred thereto.
U.S. Pat. No. 5,410,393 discloses, at FIG. 4 thereof, a transfer system in which the bias is briefly set to a first polarity just before transfer of a sheet, and then set to the opposite polarity for the duration of the transfer step.
U.S. Pat. No. 5,541,718 discloses a transfer system in which the transfer bias is altered depending on whether a sheet is being guided by one or another guide member adjacent to the transfer zone.
U.S. Pat. No. 5,598,256 discloses, at FIG. 2 thereof, a transfer system in which the strength of the transfer field is momentarily spiked between feeding the leading edge of a sheet, and transferring the leading edge of an image to be placed on the sheet.
U.S. Pat. No. 6,009,286 discloses a transfer device in which a relatively high transfer field is provided at both the leading edge and trailing edge of a sheet being transferred.
According to the present invention, there is provided a method of transferring marking material from a charge receptor to a print sheet in an electrostatographic printing apparatus. The print sheet is moved relative to the charge receptor in a process direction through a transfer zone, whereby the print sheet presents to the charge receptor a lead edge and a trail edge. When the lead edge is in the transfer zone, an initial DC bias is provided between the print sheet and the charge receptor. During the moving step, the DC bias is decreased in absolute terms to at least zero before the trail edge enters the transfer zone.